Complete cures of various tumors like leukemias, lymphomas and solid tumors by the use of chemotherapeutic agents are rare because of heterogeneous sensitivity of tumor cells to each antitumor agent. Cancer chemotherapy also fails because of intrinsic resistance of tumors to multiple drug therapies. In other cases, a tumor may become resistant to the antitumor agents used in a previous treatment. The therapeutic effects of these agents are then eliminated. An even graver problem is that recurrent cancers are resistant not only to the cancer suppressants used in previous treatments, but also manifest resistance to other antitumor agents, unrelated to the agent used previously either by chemical structure or by mechanism of action. These phenomena are collectively referred to as multiple drug resistance (mdr) and contribute widely to cancer treatment failures in the clinic.
The major documented cause of multiple drug resistance is overexpression of a membrane glycoprotein (the multiple drug transporter) responsible for pumping structurally diverse antitumor drugs from cells. See D. Houseman et al., A Molecular Genetic Approach to the Problem of Drug Resistance in Chemotherapy, 504-517 (1987) (Academic Press, Inc.); R. Fine and B. Chabner, Multidrug Resistance, in Cancer Chemotherapy 8, 117-128 (H. Pinedo and B. Chabner eds. 1986).
Tumor cells expressing elevated levels of the multiple drug transporter accumulate far less antitumor agents intracellularly than tumor cells having low levels of this enzyme. The degree of resistance of certain tumor cells has been documented to correlate with both elevated expression of the drug transporter and reduced accumulation of antitumor drugs. See M. Gottesman and I. Pastan, J. Biol. Chem. 263, 12163 (1988); see also A. Fojo et al., Cancer Res. 45, 3002 (1985). This form of multiple drug cross-resistance involves agents derived from natural products, such as the vinca alkaloids, the anthracyclines, the epipodophyllotoxins, actinomycin D and plicamycin. See I. Pastan and M. Gottesman, New England J. Med. 1388, 1389 Table 1 (May 28, 1987).
Adenocarcinomas derived from adrenal, kidney, liver, small intestine, and colon tissue are notorious for exhibiting inherent cross-resistance to chemically unrelated chemotherapeutic agents. See M. Gottesman and I. Pastan, supra at 12165; see also A. Fojo et al., J, Clin. Oncol. 5, 1922 (1987). These tissues normally express higher levels of the multidrug transporter. Other tumors documented to express high levels of the multidrug transporter include pancreatic, carcinoid, chronic myelogenous leukemia in blast crisis, and non-small cell lung carcinoma. Tumors documented to initially be drug-sensitive but to then become drug resistant include neuroblastoma, pheochromocytoma, acute lymphocytic leukemia in adults, acute nonlymphocytic leukemia in adults, nodular poorly differentiated lymphoma, breast cancer and ovarian cancers. It is estimated by the National Cancer Institute that approximately half a million tumor samples a year will be drug resistant because of aberrant levels of expression of/ the multidrug transporter. See L. Goldstein et al., Expression of Multidrug Resistance Gene in Human Cancers, J. National Cancer Institute 81, 116 (1988).
Elevated levels of expression of the mdr drug transporter in these tumors would lead to reduced intracellular levels of antitumor agents in the tumor and would cause suppression of chemotherapeutic efficacy. Tumors having elevated levels of the multiple drug transporter would require therapeutic doses of cancer suppressants far in excess of tumors exhibiting lower levels of the mdr drug transporter. Agents that inhibit the active efflux of antitumor agents by the drug transporter or agents that potentiate the efficacy of chemotherapeutic agents would enhance the activity of various antitumor agents on tumor cells. As a result of the present inventor's study, it has unexpectedly been found that when the agents disclosed herein are used together with an antitumor agent, they enhance the therapeutic effect of the antitumor agent.
A number of agents used clinically as calcium channel-blockers, calmodulin inhibitors and antiarrhythmic agents promote the activity of antitumor agents against resistant tumor cells, see Tsuruo et al., Cancer Res. 44, 4303 (1984); 43, 2267 (1983). Verapamil, caroverine, clomipramine, trifluoperazine, prenylamine, diltiazem, nicardipine, and quinidine enhance the activity of antitumor agents against resistant sublines of murine leukemia cells. Most agents potentiating the activity of antitumor agents are calcium antagonists, and the serious cardiotoxicities that arise during treatment have limited their clinical usefulness. While the inventor does not wish to be bound by any theory of operation for the present invention, it is noted that the agents disclosed herein are not known to have calcium antagonism. They have, however, been found to elevate the intracellular concentration of antineoplastic drugs in tumor cells overexpressing the multiple drug transporter. Sensitization of drug resistant tumors and elevation of intracellular antitumor drug concentrations probably occur by a mechanism different from calcium antagonism.